Operation
A classic lamp contains a standard incandescent bulb or halogen lamp which heats a tall (often tapered) glass bottle. A formula from 1968 US patent consisted of water and a transparent, translucent or opaque mix of mineral oil, paraffin wax and carbon tetrachloride. The clear water and/or mineral oil can optionally be coloured with transparent dyes.
Common wax has a density much lower than that of water, and would float on top under any temperature. However, carbon tetrachloride is heavier than water (also nonflammable and miscible with wax), and is added to the wax to make its density at room temperature slightly higher than that of the water. When heated, the wax mixture becomes less dense than the water, because the wax expands more than water when both are heated. It also becomes fluid, and blobs of wax ascend to the top of the device where they cool (which increases their density relative to that of the water) and then they descend. A metallic wire coil in the base of the bottle acts as a surface tension breaker to recombine the cooled blobs of wax after they descend.
However, lava lamps made for the US market since 1970 do not use carbon tetrachloride, because its use was banned that year due to toxicity. The manufacturer (Haggerty) states that their current formulation is a trade secret.
The underlying fluid mechanics phenomenon is a form of Rayleigh–Taylor instability.
The bulb is normally 25 to 40 watts. Generally it will take 45-60 minutes for the wax to warm up enough to freely form rising blobs, when operating the lamp at standard room temperature. It may take as long as 2 to 3 hours if the room is below standard room temperature.
Once the wax is molten, the lamp should not be shaken or knocked over or the two fluids may emulsify, and the fluid surrounding the wax blobs will remain cloudy rather than clear. Some recombination will occur as part of the normal cycle of the wax in the container, but the only means to recombine all of wax is to turn off the lamp and wait a few hours. The wax will settle back down at the bottom, forming one blob once again. Severe cases can require many heat-cool cycles to clear.
Read more about this topic: Lava Lamp
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